Method of fabricating ring shapes by hot pressing

ABSTRACT

Refractory powders are hot pressed into ring shapes of approximately theoretical density by compressing the powders at a temperature of about 1,000* to 2,000*C. under a pressure of about 200 to 5,000 pounds per square inch in a mold cavity containing a metal ring. The ring is located within the cavity at the highest stress point in the ring shape created during the pressing, and during the hot-pressing the metal ring melt-diffuses into pressed refractory powder. The resulting ring shapes are strong and essentially stress free.

United States Patent Flanagan 1 March 6, 1973 [54] METHOD OF FABRICATING RING 2,287,952 6/1942 Tormyn ..29/420 ux SHAPES BY HOT PRESSING 2,571,868 10 1951 Haller ..75/200 ux 2,706,693 4/1955 H ll ....29/l82.1 X lnvemorI Elman Flanagan, Hockessin, 2,753,s59 7/1956 BZnin ..29/182.1 x

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington Del Primary Examiner-Charles W. Lanham Assistant ExaminerDonald C. Reiley, Ill Flledi 1971 Att0rneyLynn N. Fisher 21 A i.N 114,900 1 pp 0 57 ABSTRACT [52] Us CL 75/226 29/192 R 29/420 5 Refractory powders are hot pressed into ring shapes of ZQ/DIG 6 approximately theoretical density by compressing the 4 l1 powders at'a temperature of about 1,000 to" 2,000C. [51] Int Cl Bzzf 3/24 under a pressure of about 200 to 5,000 pounds per [58] Field of Searchm29/420 420.5 192 R 192 square inch in a mold cavity containing a metal ring, 29/DIG 31 182 179 264/l11 75/20O The ring is located within the cavity at the highest stress point in the ring shape created during the pressing, and during the hot-pressing the metal ring References Cited melt-diffuses into pressed refractory powder. The resulting ring shapes are strong and essentially stress UNITED STATES PATENTS free 2,161,597 6/l939 Swartz ..29/420 UX 6 Claims, 2 Drawing Figures I I t I// sf "1:-"' I I, 7 7 e FIG.

FIG.2

PATENTED 6 E173 METHOD OF FABRICATING RING SHAPES BY HOT PRESSING BACKGROUND OF THE INVENTION late application of pressure and rapid cooling are disclosed in Hot-Pressing High-Density Small Grain Size Beryllia by R. E. Johnson, Ceramic Bulletin, Vol. 43, No. 12 (1964) at pages 886 to 888 and more recently in US. Pat. No. 3,413,392.

In hot pressing ring shapes there is a problem whereinif sufficient pressure is applied to the inner sur- 7 face of the powder ring during compaction, e.g., by a core mold, so that the inner surface does not appreciably deform, the pressure causes the ring shapes to crack after compaction because of the opposition of the core mold to contraction of the dense ring shape on cooling, the contraction being caused by the difference between the thermal expansion of the pressed ring shape and the core mold material, e.g., usually a graphite mold.

In assignees pending application Ser. No. 878,641, filed Nov. 21, 1969, and now abandoned a method for hot pressing ring shapes is disclosed. This method provides sufficient support pressure for the inner surface of the powder ring during compaction to prevent appreciable deformation yet allows contraction of the inner surface of the dense ring shape on cooling to prevent cracking. That method is based on the use of a thin-walled deformable liner inside an outer mold shell as a core mold, in combination with support means inside the deformable liner. The refractory powder is compressed in the space between the deformable liner and shell, with the liner defining the inner surface of the ring. The integrity of the deformable liner is maintained during compression by means of the support means such as a solid rod. After compression and before cooling, the support means can be removed to permit the deformable liner to deform under the contractive pressure of the cooling dense ring; or alternatively it can be maintained when its support pressure is low enough to permit deformation of the liner under the eontractive forces of the ring on cooling.

SUMMARY OF THE INVENTION 1 have discovered an improved method for hotpressing refractory powders into dense ring-shaped compacts at temperatures between 1,000 and 2,000C. and pressures between 200 and 5,000 pounds per square inch which comprises:

1. placing a metal ring into the hollow cavity of a mold assembly, the cavity being defined by the inner surface of an outer mold, the outer surface of an inner core mold and the surface of a piston, said metal ring being located within the cavity at the highest stress point in the ring shape created during the pressing;

2. loading a refractory powder into the remaining portion of the hollow cavity of the mold assembly,

3. inserting an opposed piston into the cavity of the mold assembly;

4. inserting the mold assembly into the heating zone of a hot press;

5. engaging the pistons of the mold assembly between the rams of the press;

6. heating and compressing the refractory powder and metal ring to diffuse the metal into the pressed ring shape and to obtain the desired density; and

7. cooling the resultant dense product.

For some reason not exactly understood, the meltdiffusion of the metal ring into the refractory powders during the hot-pressing, provides an essentially stressfree or stronger ring shaped compact. The process is useful with many types of refractory powders and results in a novel, useful product. Further the process of the invention results in a ring shape with one of its surfaces being under compression.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical cross-section of an embodiment of the invention illustrating a mold assembly prior to the hot-pressing step.

FIG. 2 is the same vertical cross-section after the hot pressing step.

DETAILED DESCRIPTION OF THE INVENTION The process of this invention can be applied to a variety of refractory powders such as oxides, borides, nitrides, carbides, silicides, beryllides, sulfides, mixtures of such refractory powders, and mixtures of one or more refractory powders with one or more metals such as iron, cobalt, nickel, tungsten, molybdenum, chromium, titanium, zirconium, niobium, tantalum, hafnium, their mixtures with each other, and their mixtures and alloys with other metals. Mixtures of the refractory powders with iron, cobalt, nickel, molybdenum, tungsten or mixtures thereof are particularly preferred because of their desirable physical properties.

The process is useful in compacting tungsten carbide, silicon carbide, aluminum oxide, tantalum carbide, titanium carbide, titanium nitride, aluminum nitride, and mixtures thereof when these materials are bonded with metals, e.g., cobalt, nickel, cobalt-tungsten, nickel-molybdenum and nickel-tungsten alloys, and the like. Materials particularly useful in the process of the invention are the cobalt bonded tungsten carbides of U.S. Pat. No. 3,451,791, the metal bonded nitrides of U.S. Pat. Nos. 3,409,416 and 3,409,419 particularly nickel molybdenum or nickel tungsten bonded titanium and alumina nitrides, and the metal bonded alumina-titanium carbide material of US. Pat. No. 3,542,529, e.g., nickel-molybdenum bonded alumina and titanium carbide and the nickel-molybdenum bonded titanium nitride-titanium carbides of [1.8. Pat. No. 3,671,201.

The improvement embodied in the process of the invention is the use of a metal ring in the mold cavity. This metal ring is melted during the hot pressing operation and diffuses into the refractory powder or metalrefractory powder mixture while they are being compacted. The metal ring disappears during the hotpressing, and relieves the stress within the ring that would otherwise be caused during cooling and contraction. After the hot-pressing has been completed, part of the space originally occupied by the metal ring may remain as a void.

It is important where the metal ring is located in the mold cavity. It should be located at the highest stress point created by the hot-pressing step, i.e., the ring is located at the point in the finished ring shape which will fracture due to internal stresses. When ring shapes are made by hot pressing, as the ring shape cools down, stress is created within the shape due to contraction. Depending upon the design of the ring shape, there is usually one area that will have the greatest stress. It is this area of the ring shape that will fracture during cooling, or when the ring shape is dropped.

The area where the metal ring should be located can normally be determined by a study of the cross section of the ring shape. Or, if desired, ring shapes can be hot pressed without using the metal ring, and the highest stress point can be determined by examination of where the fracture occurs.

Generally the highest stress point will be located where there are the sharpest or smallest angles between the adjacent outer surfaces of the inner core mold. However, resistance to fracture on cooling also will often be less at those parts of the ring shape with the smallest cross section. Thus in the hot pressing of some ring shapes, the ring shape may be located adjacent the inner surface of the outer mold rather than adjacent the inner core mold.

The reason why the melt-diffusion of the metal ring alleviates fracturing problems is not fully understood. The melt-diffusion of the metal ring may help by stressrelief, or the melt-diffusion of the metal may form a new material with greater strength in the immediate area where the metal ring had been located. In any event, a stronger, essentially stress-free ring shaped body is obtained.

Further the metal ring will usually not be located such that the metal ring will diffuse into the useful or working surface of the ring shape. Usually it is desirable to keep this useful or working surface homogeneous. For example, if the ring shape is to be used for jewelry, e.g., a watch case, the ring should not be located such that it will melt-diffuse into the surface that will be visible.

The size of the metal ring inserted into the mold can vary over a wide range. Broadly the metal ring should be of sufficient thickness to provide the desired strength in the pressed ring shape. However, it should not be of such a thickness that the metal will melt-diffuse into the useful or working surface. Generally the thickness of the metal ring will vary from one-third to one-twentieth of the thickness of the area of the ring shape where it is located, and usually one-fourth to one-tenth of this thickness.

The metal used for the ring must be one that will not react significantly with the mold parts, usually graphite, at the pressing temperature and contact times involved in the process. The metal must also be one that will melt-diffuse into the refractory powders during the processing step. With the previously set forth refractory materials and their mixtures with metals, the metal can be iron, an iron based alloy, e.g., steel or stainless steel, cobalt, cobalt alloys, nickel, nickel alloys or mixtures thereof.

Optimum temperatures, pressures and times of application for the various refractory powders and their mixtures with metals will vary and should be such that there is a substantially complete diffusion of the metal ring into the compacted ring shape. The pressing temperature should exceed the lowest eutectic temperature of the metal of the ring with the refractory powders adjacent thereto.

In many embodiments it is desired during the hot pressing step to first apply a low pressure to the refractory powders. This low pressure is maintained during a heat-up and a hold period to force the material to conform to the mold. The material is then hot pressed for a short time by applying a high pressure.

When used, the initial or low pressure applied to the refractory material will range between 50 and 1,000 pounds per square inch. This pressure is applied for 1 to 10 minutes as the powder heats up to the pressing temperature and for a hold period of up to 10 minutes in .order to force the powder sample to fill the mold cavity.

The final pressures applied during the process of the invention generally range from a minimum of about 1,000 pounds per square inch to 5,000 pounds per square inch for 10 seconds to 10 minutes, but in the case of operating at lower temperatures, e.g., 1,000C. with molybdenum molds, pressures up to 30,000 pounds per square inch could be used, although these are generally not necessary. The pressure used at the required temperature must be sufficient to compact the material to a density of at least percent of theory for the refractory composition involved and preferably in excess of 99 percent. In the most preferred case, the

sample will be compressed to a density of 100 percent of theory.

The temperatures produced generally range from about 500C. to 2,500C., and under most operating conditions a temperature between 1,000C. and 2,000C. is required to fabricate true refractory materials to high density. The materials of construction of the hot press generally impose a maximum temperature of about 2,000C., since above this temperature most of the materials used lack sufficient strength.

In many embodiments of the process of this invention, depending upon the refractory powders being used and the pressing conditions, it will be desirable to perform some or all of the steps in the absence of oxygen. An oxygen free environ-ment can be obtained by methods known to the art such as locating the equipment within a sealed housing and maintaining a vacuum or an inert gas atmosphere within the housing.

The metal ring and refractory powder are located within the desired mold cavity. This cavity is formed by the inner surface of an outer mold, the outer surface of an inner mold, and two opposite pistons. As is apparent to those skilled in the art, the above description of the mold cavity could be varied without departing from the process of the invention; all that is necessary is that the mold be useful for forming ring shapes. Thus if desired, the inner core mold and one of the pistons could be an integral unit.

A useful mold assembly is set forth in the Figures and will now be described.

Referring to FIG. 1, in this embodiment of the invention, the sample of refractory powder 1 is in the space between the outer mold 2, an integral inner core mold and lower piston 3 and a hollow top piston 4. The metal ring 5 is located within mold cavity at the highest stress point created by the pressing, i.e., in this embodiment, adjacent the inner core mold. The powder can be loaded by simply pouring it into the cavity formed by the outer mold, the integral inner core mold and lower piston before the insertion of the hollow top piston. The pistons slide in the space between the outer mold and with each other. The refractory powder is compacted by the application of pressure in the direction of the arrows, transmitted through the press rams 6, which are activated by means such as pneumatic or hydraulic jacks or presses.

FIG. 2 illustrates the mold assembly after the hotpressing step has been completed. The refractory powder 1 has now been compacted and the metal ring has disappeared. Instead the metal has melt-diffused 7 into the compacted refractory powder. In this embodiment, the melt-diffusion of the metal ring is illustrated as having created a void where the ring was located.

The shape and dimensions of the mold can vary as will be apparent to those skilled in the art. Thus the outer mold can have an interior shape that is round, square, elliptical or any other practical shape, the inner core mold's outer surface may have a corresponding or different shape from that of the outer mold, but is usually round.

The mold assembly parts can be made of any suitable refractory material which has good strength at high temperatures. Representative of suitable materials are alumina, zirconia, beryllia, silicon carbide, boron nitride, boron carbide, zirconium carbide, molybdenum, tungsten, tungsten carbide, titanium carbide, tantalum carbide, titanium borid-e, various mixtures of these materials, and graphite. While the material used depends to a large extent upon the size of the operation and the pressures and temperatures that will be used in pressing the powder, it is commonly preferred to use graphite for the mold parts.

The dense compact can be cooled slowly or rapidly, such as by leaving it in the heated zone of the hot press or by immediately removing it. It is frequently preferred to cool the compact very rapidly once densification is complete.

The method and apparatus of this invention can be utilized in a single unit hot-pressing operation or they can be optimized by the rapid procedures disclosed in US. Pat. No. 3,413,392. It is similarly possible to utilize horizontal as well as vertical hot-press techniques as well as multiple molds in a single press. In the process of the invention the mold assembly is inserted into a heating susceptor or heating zone of a hot-press, if desired it can be pre-heated, or it can be heated in place by inductive heating, dielectric heating, resistance heating, plasma torch, hot vapors or any other means known to the art.

The method and apparatus of this invention are further illustrated in the following examples wherein parts and percentages are by weight unless otherwise noted.

EXAMPLE 1 Within a mold cavity designed to form a ring is loaded l,060 parts of titanium carbide, 2,400 parts of titanium nitride, 250 parts of nickel and 290 parts of molybdenum.

These powders have been prepared by charging a 2.6 gallon Sweco Vibro-Energy" mill with the indicated amount of titanium carbide in the form of a 2-4 micron powder, titanium nitride of 325 mesh size, a nickel powder (Mond No. B-287 of International Nickel Corporation) and a molybdenum powder (Grade 390/100, Sylvania Electric Products). Also charged into the mill is 160 parts of cylindrical sintered alumina grinding media. After all the ingredients have been added, the mill is operated for 24 hours.

Previously a steel ring having one-fourth the thickness of the desired ring shape has been placed within the mold adjacent the outer mold surface. The mold is gently tapped while charging the powder, to pack and distribute it evenly. An upper piston is then fitted into the mold and the entire assembly is placed in the heat zone of a hot press, the mold being held in a vertical position between the rams of the press.

Pressure is applied to the rams to compact the sample under a pressure of 500 pounds per square inch, the temperature of the furnace is brought to 1,400C., and the pressure on the sample is increased to 4,000 pounds per square inch. After twelve minutes at l,700C. and 4,000 pounds per square inch, the furnace is shut off and the rams are withdrawn. After being allowed to cool, the mold is removed from the furnace and the sample is force out of the mold.

The steel ring is no longer present and by visual observation it can be noted that the steel has melt-diffused into the ring shape. The resulting ring is accurately molded to the desired dimensions and requires very little finishing prior to use. Further it is strong, essentially stress-free, and can be used as a die.

EXAMPLE 2 A ring, suitable for polishing to give a wrist watch case, is fabricated by hot pressing a composition consisting of 3,000 parts of titanium nitride, 240 parts of aluminum nitride, 1,050 parts of nickel and 400 parts of molybdenum in a mold containing a steel ring adjacent the inner core mold. The graphite mold assembly is similar to that illustrated in FIG. 1. The ring shape to be pressed as an outside diameter of two inches, an inside diameter of one inch and an average thickness of three-eighths of an inch. The metal ring used has a thickness of one-eighth of an inch.

The powder mixture is prepared from the following materials:

-325 mesh grade titanium nitride powder having a specific surface area by nitrogen adsorption of 1.1 square meters per gram, available from Materials for Industry Inc.;

325 mesh grade aluminum nitride powder having a specific surface area by nitrogen adsorption of 2.3 square meters per gram, available from Materials for Industry Inc.;

Fine nickel powder having a specific surface area of 0.48 square meters per gram, available from International Nickel Co.; and Fine molybdenum powder sold as Grade 390/100 by Sylvania Electric Products.

These constituents are milled for 24 hours in a mill using cylindrical sintered alumina as the grinding media. The milled materials are recovered from the mill, washed with hexane and dried under vacuum. The dry powder is then passed through a mesh screen, in a nitrogen atmosphere.

The powder prepared as described above is charged to the mold and hot pressed as described in Example 1. After five minutes at 1,400C. and 4,000 pounds per square inch the mold assembly is removed from the hot zone and allowed to cool rapidly. The sample is then forced out of the mold by applying pressure and the resulting watch case then requires only surface polishing with diamond abrasive in order to develop an attractive appearance. The steel ring has disappeared and the watch case is strong and essentially stress free.

EXAMPLE 3 The procedures set forth in Example 2 are followed except that the refractory powder used has the following composition:

3,000 parts of titanium nitride 275 parts of alumina 310 parts of nickel 350 parts of molybdenum the alumina being a fine (-325 mesh) alpha alumina powder powder commercially available as Alcoa Superground Alumina XA-l6.

lclaim:

1. An improved method for hot-pressing refractory powders into dense ring-shaped compacts at a temperature of between 1,000 and 2,000C. and a pressure between 200 and 5,000 pounds per square inch comprising l. placing a metal ring into the hollow cavity of a mold assembly, the cavity being defined by the inner surface of an outer mold, the outer surface of an inner core mold and the surface of a piston, said metal ring being located within the cavity at the highest stress point in the ring shape created during the pressing;

2. loading a refractory powder into the remaining portion of the hollow cavity of the mold assembly,

3. inserting an opposed piston into the cavity of the mold assembly;

4. inserting the mold assembly into the heating zone of a hot press;

5. engaging the pistons of the mold assembly between the rams of the press;

6. heating and compressing the refractory powder and metal ring to diffuse the metal into the pressed ring shape and to obtain the desired density; and

7. cooling the resultant dense product.

2. The method of claim 1 wherein the ring-shaped compact is a watch case and the metal ring is an iron based alloy.

3. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is cobalt bonded tungsten carbide.

4. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is nickel-molybdenum or nickeltungsten bonded titanium nitride and aluminum nitride.

5. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is nickel-molybdenum bonded alumina and titanium carbide.

6. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is nickel-molybdenum bonded titanium carbide and titanium nitride. 

1. An improved method for hot-pressing refractory powders into dense ring-shaped compacts at a temperature of between 1,000* and 2,000*C. and a pressure between 200 and 5,000 pounds per square inch comprising
 1. placing a metal ring into the hollow cavity of a mold assembly, the cavity being defined by the inner surface of an outer mold, the outer surface of an inner core mold and the surface of a piston, said metal ring being located within the cavity at the highest stress point in the ring shape created during the pressing;
 2. loading a refractory powder into the remaining portion of the hollow cavity of the mold assembly,
 2. The method of claim 1 wherein the ring-shaped compact is a watch case and the metal ring is an iron based alloy.
 3. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is cobalt bonded tungsten carbide.
 3. inserting an opposed piston into the cavity of the mold assembly;
 4. inserting the mold assembly into the heating zone of a hot press;
 4. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is nickel-molybdenum or nickel-tungsten bonded titanium nitride and aluminum nitride.
 5. The process of claim 1 wherein the dense product is a watch case, the metal ring is an iron based alloy and the refractory powder is nickel-molybdenum bonded alumina and titanium carbide.
 5. engaging the pistons of the mold assembly between the rams of the press;
 7. cooling the resultant dense product. 